Recombinant systems has been widely used to produce a variety of molecules including therapeutic proteins, vitamins and other dietary supplements. Plants, for example, are an extremely valuable source of essential dietary nutrients, such as vitamins. However, the expanding global human population is placing increasing strain on the ability of crop resources to meet nutrition and healthcare demands. Hence, there exists a very real need to increase production of specific molecules, for instance to increase the nutritional and economic value of crops. In addition, physiological traits that enhance a crop's growth characteristics, or its competitive edge in the face of adverse climate or pathogens, are also of economic value.
The vast majority of agronomic traits are quantitative and are controlled polygenetically. For example, the high-value nutrient α-tocopherol (vitamin E) is the end product of a complex series of chemical and enzymatic events, rather than the product of a specific gene (indeed, α-tocopherol synthesis is known to involve the action of at least 16 enzymes. See, e.g., Munne-Bosch & Alegre (2002) Critical Reviews in Plant Sciences 21:31-57. Genetic engineering to maximize the synthesis of such products will likely require increasing the expression of several genes central to its metabolic pathway.
At present, the commonly used technology for increasing the level of a product is the introduction of cDNA encoding the protein of interest. Overexpression of a protein that is the rate-limiting factor in a synthetic pathway may give some increase in product synthesis, but this process is limited by secondary kinetic bottlenecks. While it might be desirable to simply add further cDNAs to overcome such barriers, there are several technical limitations that render this conventional approach to multigenic engineering inappropriate when the regulation of many genes is required:                (i) The efficiency of transgene integration decreases with increasing size of the targeting construct. Thus, the insertion of several (e.g., more than 5, 8 or even 10) cDNAs and promoters into a single targeting construct would, in many cases, have a significant negative impact upon the efficiency of integration.        (ii) Inserting multiple transgenes into cell line as individual DNA constructs requires a different selection marker for each new gene. However, there is a limit to the number of different selection markers available—especially where industry/consumer concerns over the use of antibiotic markers is an issue.        (iii) There are a limited number of characterized promoters available to drive the expression of cDNAs. For instance, repeated promoter use may lead to gene silencing in plants, which do not tolerate repetitive regulatory sequences well.        (iv) Repeated transformations or cross breeding to insert all the cDNAs would be very time consuming.        
In some cases it may feasible to utilize the above approach to multigenic regulation for a very limited number of genes in a pathway (two or three at most). Indeed, this approach has been used successfully to increase the level of β-carotene (the precursor to vitamin A) in rice. See, e.g., Ye, X. et al. (2000) Science 287:303-5.
Thus, when faced with the challenge of simultaneously regulating many (e.g., ten or even more) genes in order to maximize the level of production, existing technologies fall well short of delivering this capability.
Therefore, there remains a need for compositions and methods for overexpressing multiple genes in a target cell or organism. Such methods would open up vast new economic opportunities, for example in agronomy.